Field of the Invention
The present invention relates to an optical scanning apparatus that includes a light deflector.
Description of the Related Art
As discussed in Japanese Patent Application Laid-Open No 2013-114100, an image forming apparatus (e.g., a laser beam printer, a digital copying machine, or a digital FAX) can be configured to include an optical scanning apparatus that can perform optical writing with a laser beam. Many of the optical scanning apparatuses include an optical box that supports a plurality of optical components, including a light deflector. The optical box includes an aperture through which the laser beam can pass.
According to the configuration discussed in Japanese Patent Application Laid-Open No. 2013-114100, there will be a possibility that toner particles, paper powder, and aerial dust may enter the optical box via a through-hole of the optical box or a clearance between the optical box and a lid. If a polygon mirror serving as the light deflector rotates at a higher speed in this case, fine particles will adhere to an edge portion of each reflection surface in a main scanning direction. FIG. 8 illustrates a polygon mirror to which fine particles have adhered. When the polygon mirror 4 rotates in a clockwise (CW) direction (seen from the above) indicated by an arrow A, a negative pressure is generated on the left side of each reflection surface 12 and therefore fine particles are dragged toward the left side. As a result, the fine particles adhere to a range Y1. Further, a positive pressure is generated on the right side of each reflection surface 12 and fine particles are struck onto the reflection surface 12. As a result, the fine particles adhere to a range Y2. In FIGS. 8, H1 and H2 indicate deflection positions of a laser beam concentrated on the reflection surface of the polygon mirror toward a sub scanning direction. The deflection position shifts from the position H1 to the position H2 when the polygon mirror rotates. The light beam deflected in the vicinity of the position H1 forms an image in the vicinity of a writing start point of an image region. The light beam deflected in the vicinity of the position H2 forms an image in the vicinity of a writing finish point of the image region. Therefore, the fine particles having adhered to the range Y1 undesirably reduces the quantity of light in the vicinity of the writing start point of the image region. The fine particles having adhered to the range Y2 reduces the quantity of light in the vicinity of the writing finish point of the image region. The reduction in reflection rate at both edge portions of each reflection surface 12 causes a significant amount of unevenness in the image density.